1. Field of the Invention
The present invention relates to acoustic generators, in particular Helmholtz resonators.
2. Description of Related Art
It has always been desirable to control the noise, particularly the audible noise of engines or machines. Noise control can increase the comfort and experience of riding in an airplane or an automobile. Noise control and cancellation is typically achieved in flow ducts by generating a sound wave that is of the same amplitude and frequency, yet 180.degree. out of phase with the noise. Through superposition, the generated sound wave subtracts from the noise to eliminate or reduce the intensity of the undesirable sound.
Present active noise reduction systems typically have a sound transducer connected to a computer. The transducer and computer sense and digitize the noise. The computer is also connected to a sound generator. The computer analyzes the noise, and provides signals to drive the sound generator to create a sound wave that will limit or cancel the propagation of the noise. With the advancement of analytical techniques, very substantial noise reduction can now be accomplished.
One type of noise generator used in noise cancellation systems is an active Helmholtz generator. A Helmholtz generator typically has a housing with an opening that allows air flow between an internal housing cavity and the region external to the opening. Sound is created in active Helmholtz generators by displacing the air within the cavity, which moves the air within the opening. The Helmholtz resonator can be modeled as a mass/spring assembly, wherein the air within the cavity functions as a spring, and the air within the opening region as an oscillating mass. Each Helmholtz resonator therefore has a resonant frequency at which the highest intensity of audio sound is generated for a given input energy. Additionally, the Helmholtz resonator contains very high frequency acoustic resonance within the cavity of the resonator. Unfortunately, the frequency band of a single Helmholtz resonator, being a tuned system, is somewhat narrow, thereby limiting the frequency range of the noise that can be canceled in an active noise control system.
U.S. Pat. No. 4,413,198 issued to Bost discloses a sound generator that includes a piezoelectric transducer coupled to a pair of Helmholtz resonators. The resonant frequency of each Helmholtz resonator is different from the resonant frequency of the transducer. When the transducer is excited, the sound generator can provide sound of substantial intensity across the range of the three resonant frequencies of the transducer and resonators.
Further increasing the frequency range of sound using the Bost device would require the inclusion of additional housing units each with varying resonant frequencies. Providing extra units would add to the bulk, weight and cost of the overall system. It would therefore be desirable to have a sound generator that was compact, light and was capable of providing a broad frequency range of sound waves.